This invention relates to a method of increasing the viscosity of epoxide resin compositions having reduced flow.
It has long been known that epoxide resins, i.e., materials containing on average more than one 1,2-epoxide group, are highly desirable for use in gel coats and other "high build" (i.e., thick) coatings. Thixotropic compositions that are readily applied but which resist running and sagging, whereby formation of resin-starved areas is avoided, are particularly useful. Thixtropy can be induced by addition of silica or asbestos, but the incorporation of these materials liberates dusts and so causes handling problems. Their use has therefore been discouraged on environmental health grounds. Further, the thickening agents are incorporated into the resin or its hardener, or both, and an immediate increase in viscosity results. Therefore, when the resin and its hardener are mixed, equipment capable of handling viscous liquids must be used. Whilst this is perfectly satisfactory for many applications, there are occasions, particularly in civil engineering applications, when mixing must take place "on site," that is, away from special mixers and often outdoors, and hence possibly at low temperatures when the liquids will be even more viscous. To overcome the difficulty of mixing viscous liquids on site, methods have been proposed whereby only the mixture of resin and hardener is viscous or thixotropic, the individual components being free flowing.
British Patent Specification No. 1 230 334, for example, describes a process in which an increase in viscosity was effected by an in situ reaction between an aliphatic primary monoamine and an aliphatic or aromatic diisocyanate. According to British Patent Specification No. 1 454 388, an increase in viscosity may be achieved by in situ reaction between a primary polyamine, optionally containing a secondary polyamine, and a monoisocyanate, optionally with a diisocyanate.
Such formation of a polyurea results in thixotropic mixtures but it suffers from several drawbacks. It is extremely sensitive to slight changes in the quantities of amine and isocyanate, to variations in temperature, and to conditions used in mixing the components. In addition, if used with epoxide resins, the components cannot be supplied as a two-part pack since the isocyanate will react with both the epoxide resin and the amine. Such compositions have not met, therefore, with commercial success.
It has now been found that the viscosity of epoxide resins, alone or in admixture with a curing agent, may be increased to give thick or jelly-like liquids by incorporation of an organo-orthosilicate, an at least moderately strong base, and water. The effect is not merely one of thickening, since, on being stirred rapidly, the mixture of epoxide resin, organo-silicate, base, and water exhibits a strong reduction in viscosity, i.e., the mixture is thixotropic. It is believed, although the utility of this invention does not rely on the truth of this belief, that the base catalyses hydrolysis of the organo-orthosilicate by the water, resulting in rapid formation of hydrated silica in the resin and hence a rapid increase in the viscosity of the mixture.
This method overcomes many of the problems encountered with prior processes for bringing about an increase in viscosity. The compositions cause no dust hazard, they may be supplied as two part mixtures (since the silicate does not react with the epoxide resin), and the amount of silicate added is not especially critical. When such mixtures contain insufficient of the base to cure the epoxide resin they are stable on storage for prolonged periods yet, when mixed with a curing agent for epoxide resins, they cure readily. Such mixtures containing sufficient base to cure the epoxide resin are ideally suited for on-site mixing and curing.
Incorporation of organo-orthosilicates in epoxide resin compositions is not, in itself, new.
In USSR Pat. No. 421 965 (Chemical Abstracts Vol. 81, No. 140195q) compositions are described which have improved adhesion to slag glass-ceramic floorings and which contain an epoxide resin, a polyethylene polyamine as hardener, a liquid rubber, and tetraethoxysilane. In USSR Pat. No. 514 005 (Chemical Abstracts Vol. 85, No. 79056j) there are described epoxide compositions for filling seams in slag glass-ceramic floor tiles, containing an epoxide resin, butyl methacrylate, diethylenetriamine, and tetrakis(furfuryloxy)silane. In Japanese Published Patent Application No. 108450/1980 (Derwent C.P.I. Abstract No. 70390C) compositions curing at room temperature are described which contain an epoxide resin, a hardener such as diethylenetriamine, diethylaminopropylamine, or an amine-ethylene oxide adduct, and an organic silicon compound containing an alkenoxy or cycloalkenoxy group attached directly to silicon.
USSR Pat. No. 779 338 (Derwent C.P.I. Abstract No. 54758D) discloses polymeric concrete compositions comprising (i) an epoxide resin, (ii) ethylenediamine as a curing agent for (i), (iii) ethyl silicate, (iv) water, and (v) metallised carbon fibre as electroconductor. The compositions generally contain, per 100 parts of weight of (i), 7 to 14 parts of (ii), 15 to 100 parts of (iii), 3 to 6 parts of (iv), and 10 to 50 parts of (y), together with 125 to 500 parts of a mineral filler. The compositions are for use in coatings and the manufacture of electric heaters, and the incorporation of (iii) together with the choice of the particular curing agent, i.e., (ii), and electroconductor, i.e., (y), is said to result in a composition with increased conductivity.
Japanese Published Patent application No. 56-125465 (Derwent C.P.I. Abstract No. 84394D) discloses coating compositions comprising, by weight,
(vi) 100 parts of an orthosilicate of formula R.sup.1.sub.2 Si(OR.sup.2).sub.2, R.sup.1 Si(OR.sup.2).sub.3, or Si(OR.sup.2).sub.4, where R.sup.1 denotes alkyl of 1 to 5 carbon atoms or phenyl and R.sup.2 denotes alkyl of 1 to 3 carbon atoms or a hydrogen atom,
(vii) 5 to 25 parts of an epoxide resin,
(viii) 30 to 60 parts of an aminosilane of formula H.sub.2 NR.sup.3 Si(R.sup.1)(OR.sup.2).sub.2, H.sub.2 NR.sup.3 Si(OR.sup.2).sub.3. H.sub.2 NR.sup.4 NHR.sup.3 Si(R.sup.1)(OR.sup.2).sub.2, or PG,6 H.sub.2 NR.sup.4 NHR.sup.3 Si(OR.sup.2).sub.3, where R.sup.3 denotes alkyl of 1 to 5 carbon atoms and R.sup.4 denotes alkyl of 1 to 3 carbon atoms or acyl,
(ix) 7 to 8 parts of a polyamine of formula H.sub.2 N(R.sup.5).sub.n NH(R.sup.5).sub.n NH.sub.2 or H.sub.2 N(R.sup.5 NH).sub.n R.sup.5 NH.sub.2, where R.sup.5 denotes alkyl of 1 or 2 carbon atoms and n is 1 to 6,
(x) 5 to 15 parts of water,
(xi) 50 to 150 parts of solvent, and
(xii) 2 to 7 parts of an acid catalyst.
In none of the references is there mentioned the possibility of increasing the viscosity of the mixtures and formation of thixtropy.